Aunt Ester appears as a guardian or a spiritual guide for the other characters in the play. They come to her when they feel low and are seeking spiritual cleansing, refreshment, and guidance. She is able to take them back into their pasts and help them come to terms with their own actions and decisions. Her power comes from her deep understanding and connection to the African-American experience in America.
The wisdom that Aunt Ester passes down is drawn from her own experiences and from those of her people. She was born when the first slave ships left Africa for America and she is still thriving; she carries the entire collective knowledge of what her people suffered and their struggle for freedom. This deep historical understanding allows her to guide and succor others. She can draw from the past to help people better understand their present. She is the connection between the past and the future.
August Wilson likely wanted the character to be deeply mystic and to have otherworldly knowledge. He wanted there to be no question that she was a spiritual guide, leader, and healer; by giving her an exceptionally long life where she persisted from the beginning of slavery to post-Emancipation, he ensures that she is connected to the entire history of her people.